A normal ear transmits sounds as shown in FIG. 1 through the outer ear 101 to the tympanic membrane 102 which moves the ossicles of the middle ear 103 that vibrate the oval window 106 and round window 107 membranes of the cochlea 104. The cochlea 104 is a long narrow duct wound spirally about its axis for approximately two and a half turns. The cochlea 104 forms an upright spiraling cone with a center called the modiolar where the spiral ganglion cells of the cochlear nerve 105 reside. In response to received sounds transmitted by the middle ear 103, the fluid-filled cochlea 104 functions as a transducer to generate electric pulses which are transmitted by the cochlear nerve 105 to the brain.
Hearing is impaired when there are problems in the ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea. To improve impaired hearing, auditory prostheses have been developed. For example, when the impairment is related to operation of the middle ear, a conventional hearing aid, a middle ear implant, or a bone conduction implant may be used to provide acoustic-mechanical stimulation to the auditory system in the form of amplified sound. Or when the impairment is associated with the cochlea, a cochlear implant with an implanted stimulation electrode can electrically stimulate auditory nerve tissue with small currents delivered by multiple electrode contacts distributed along the electrode.
U.S. Patent Publication 20070191673 (incorporated herein by reference in its entirety) describes one type of bone conduction implant that delivers a mechanical vibration signal to the cochlea for sound perception in persons with conductive or mixed conductive/sensorineural hearing loss. An implanted bone conduction transducer is affixed beneath the skin to the temporal bone. In response to an externally generated electrical communications signal, the transducer couples a mechanical stimulation signal to the temporal bone for delivery by bone conduction to the cochlea for perception as a sound signal. A certain amount of electronic circuitry also is implanted with the transducer to provide power to the implanted device and at least some signal processing which is needed for converting the external electrical communications signal into the mechanical stimulation signal and mechanically driving the transducer.
Bone conduction implant systems that have the vibration driving unit in the external device face the problem that the external device itself, which is magnetically held, also vibrates. That makes the external device more prone to fall off the patient than the external portions of cochlear implant systems. There needs to be a delicate matching of the amount of magnetic attraction force that holds the external device over the implant, together with the amount of vibration force needed for hearing perception. This matching has been attempted in prior art devices by using magnets in the external device which can be moved closer to or further away from the implantable magnet so as to adjust the amount of overall magnetic force. In other prior art arrangements, a stack of magnets was used in the external device rather than just a single magnet. Depending on the magnetic force that is actually needed, one or more of the magnets are used.